Motor vehicles and other large and expensive machines are often supplied (or retrofitted) with security systems to prevent unauthorized use or theft. Most motor vehicle security systems make use of one or more electrical switches mounted in, for example, the vehicle's doors, hood, or trunk. Typically, these switches are electrically connected to a central alarm circuit by one or more wire conductors. When a door, the hood or the trunk is opened, the switch mounted therein creates a connection between a conductor and the body of the vehicle, which serves as an electrical ground. This ground connection is detected by the central alarm circuit which, if the alarm is armed, sounds an alarm, e.g., the vehicle's horn.
Some security systems also include a shock detector which detects mechanical shock imparted to the body of the vehicle. Ideally, a shock detector will sound the alarm when a thief or vandal mechanically shocks the vehicle by, for example, breaking a window. In this situation, a shock detector will sound the alarm even though the door, hood and trunk switches have not been activated, improving security.
One known drawback of shock detectors is that they are frequently over-sensitive; often a mere bump of the vehicle by a pedestrian or a neighboring car door can generate a false alarm. A second drawback is that shock detectors are typically included in only the most expensive security systems, and as a result are often unavailable to owners of mid-price range vehicles.